Rare—earth element—iron—boron base magnets are widely used for voice coil motors (VCM) of hard disk drives, magnetic circuits of magnetic resonance imaging (MRI), and the like. In recent years, the applicability has been expanded to driving motors of electric cars. In particular, the heat resistance is required in the automobile use, and a magnet having a high coercive force is required to avoid high-temperature demagnetization at an environmental temperature of 150° C. to 200° C.
A Nd—Fe—B base sintered magnet has a microstructure in which principal Nd2Fe14B compound phases are surrounded by a Nd-rich grain boundary phase, and component compositions, sizes and the like of these principal phase and grain boundary phase play important roles in exerting a coercive force of a magnet. In general sintered magnets, high coercive forces are exerted by containing about a few percent by mass to ten percent by mass of Dy or Tb in magnet alloys and taking the advantage of the magnetic properties of a Dy2Fe14B compound or a Tb2Fe14B compound having an anisotropic magnetic field larger than that of the Nd2Fe14B compound. However, there is a problem in that the saturation magnetization is decreased sharply and, thereby, the maximum energy product ((BH)max) and the remanent magnetic flux density (Br) are reduced as the content of Dy or Tb is increased. Furthermore, since Dy and Tb are rare resources and are expensive metals costing a few times as much as Nd does, the usage thereof must be reduced.
In order to improve the coercive force of the Nd—Fe—B base sintered magnet while a decrease in the remanent magnetic flux density is suppressed, it is desirable to magnetically strengthen crystal grain boundaries and a magnet surface layer, which tend to become generation sources of reverse magnetic domains, by cleaning. It is known that the presence of Dy, Tb, and the like in the grain boundary phase on a priority basis rather than in the principal Nd2Fe14B phase is effective.
For example, in known methods, an alloy primarily containing Nd2Fe14B and an alloy containing a high proportion of Dy and the like are prepared separately, each powder is mixed at an appropriate ratio, and molding and sintering are conducted so as to improve the coercive force in the production of a sintered magnet (Patent Documents 1 and 2 and Non-Patent Document 1).
There are methods in which any scheme during a production process of a sintered magnet is not used, but a treatment of the resulting sintered material is conducted. In the reported methods, a rare-earth metal is introduced into the surface and a grain boundary phase of a minute and fine Nd—Fe—B base sintered magnet molded material so as to recover the magnetic properties (Patent Documents 3 and 4), or a Dy or Tb metal is applied by sputtering to a surface of a magnet processed into a small size, and a high-temperature heat treatment is conducted so as to diffuse Dy or Tb into the inside of the magnet (Non-Patent Documents 2 and 3). In addition, there is a method in which Dy is diffused into grain boundaries of a Nd—Fe—B base sintered magnet. A method in which a sputtered film is heated (Patent Document 5) and a method in which a fine powder of an oxide or a fluoride of Dy is applied to a magnet and, thereafter, a surface diffusion treatment and an aging treatment are conducted (Non-Patent Document 4) have been reported.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 61-207546
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 05-021218
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 62-74048
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2004-296973
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 01-117303
[Non-Patent Document 1] M. Kusunoki et al. 3rd IUMRS Int. Conf. On Advanced Materials, p. 1013 (1993)
[Non-Patent Document 2] K. T. Park et al. Proc. 16th Workshop on Rare Earth Magnets and Their Application, Sendai, p. 257 (2000)
[Non-Patent Document 3] Machida et al. Japan Society of Powder and Powder Metallurgy Heisei 16 Nendo Shunki Taikai Kouen Gaiyoshu (Summary of Fiscal 2004 Spring Meeting), p. 202 (2004)
[Non-Patent Document 4] H. Nakamura, IEEJ Journal, Vol. 124, No. 11, pp. 699-702 (2004)